A vehicle powertrain typically includes a transmission and an engine. The transmission transfers engine torque to a driveshaft, which rotates the wheels of the vehicle. A vehicle speed sensor determines the rotational speed of the driveshaft. One particular type of vehicle speed sensor is a Hall-effect sensor that determines the driveshaft speed based on an induced current generated by changing magnetic fields. The Hall-effect sensor does not require an external power source. However, the Hall-effect sensor fails to determine low rotational speeds accurately.
A power operated vehicle speed sensor (VSS) allows for increased accuracy in determining low rotational speeds. This type of sensor may fail when power is interrupted to the sensor. However, when the VSS is not responding, it may be difficult to determine whether power is interrupted to the VSS or the output shaft is actually motionless.